Scallops thermidor from Fish etc.: the ultimate book for seafood lovers by Mark Hix

Ingredients

• 200 ml fish stock (or ¼ good-quality fish stock cube dissolved in that amount of hot water)
• 4 shallots, finely chopped
• 100 ml white wine
• 2 tsp English mustard
• 350 ml double cream
• 200 g spinach, stalks removed
• good knob of butter
• 40 g Cheddar cheese, grated
• 12 medium-sized scallops, cleaned and the shells reserved
• 20 g freshly grated Parmesan cheese
• salt and freshly ground white pepper
• 1 egg yolk
• 1 tsp chopped tarragon
• 1 tbsp olive oil